1. Field of the Invention
The present invention relates to a filter and a radio communication device using the same.
2. Description of the Related Art
In general, a filter to limit a frequency band for a radio communication system is structured by resonant units connected in cascade. Each resonator provided in the resonant unit includes an inductor and a capacitor and adds a resister for taking account of influence of a loss. A filter of such a type can determine a frequency range of a passband and a reduction amount of a blocking band by appropriately determining an inter-resonator coupling coefficient between resonators and determining a value of external Q to indicate an amount exciting the resonator in an input unit and an output unit.
On the other hand, Q (unloaded Q) to be determined by a dielectric loss, a conductor loss and a radiation loss of the resonator is an important parameter for realizing a filter property having a steep skirt property required by a band-pass filter, etc. The dielectric loss depends on a loss property of a dielectric substrate, the conductor loss depends on a loss property of a conductor and the radiation loss depends on a resonator layout. At a relatively low frequency dominated by the conductor loss, the influence of the radiation loss is small even when each resonator is coupled in any manner. In contrast, at a relatively high frequency dominated by the radiation loss, if the conductor is placed in the vicinity of a current maximum point of the resonator, the conductor becomes a dominant factor of radiation and finally, becomes a factor to deteriorate the filter property.
As for an example of a most general filter, a filter using a resonator formed of microstrip lines has been widely known. An electromagnetic wave propagating on the microstrip line propagates while reflecting repeatedly at open end portions thereof. Accordingly, in a half-wavelength resonator formed of a microstrip line of which the electric length is a half-wavelength (180°), a standing wave of a current distribution has nodes at both ends of the microstrip line and only one antinode at a center thereof.
A filter arranging half-wavelength hairpin resonators formed of hairpin-microstrip line in cascade so as to miniaturize its size is disclosed in G. L. Matthaei, et.al, “Hairpin Comb Filters for HTS and Other Narrow-Band Applications”, IEEE MTT Trans., Vol. 45, No. 8, August 1997 (document 1).
On the other hand, a half-wavelength resonator using two straight lines and a microstrip line having an arc of a circle portion disposed between the straight lines and a filter using the resonator are disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2003-46304 (document 2). The two linear lines are designed smaller than the width of the linear line in interval there between.
In each half-wavelength resonator, the center of a microstrip line of the resonator is the antinode of a current distribution, namely the current maximum point. Accordingly, in a filter in which a plurality of half-wavelength resonators arranged by shifting them by quarter-wavelength (90°), an end portion of a microstrip line of the next resonator is close to the current maximum point, so that the radiation at the maximum point becomes larger. According to the filter layout which is disclosed in the document 1 and in which the half-wavelength hairpin resonators are arranged in cascade, current maximum points that are folding portions of the microstrip lines of each resonator close to one another among the adjacent resonators. Therefore, radiations from the folding portions are increased. Like this, when the radiation losses of the resonators become large, it becomes hard to realize a filter property having a steep skirt property resulting from increases in Q values of the resonators.
On the other hand, relative magnitude correlation between the conductor loss and the radiation loss depends on a frequency of an electromagnetic wave propagating on the microstrip line. As mentioned above, in a low-frequency band, although the conductor loss is dominant, the relative magnitude correlation is inverted gradually as the frequency becomes higher, and in a high-frequency band, the radiation loss is apt to become dominant. Since the conductor loss is an energy loss caused from an electric resistance component of the conductor (conductor to form strip and ground plane) of the microstrip line, the conductor loss tends to become further dominant in accordance with an increase in its resistance component.
A resonator using a conventional microstrip line has a resonant frequency in a band of, for example, not higher than 3 GHz, and the conductor loss of which is dominant, because the resistance component of the conductor is relatively large. The conductor loss is reduced with relative ease by giving a uniform of a current density distribution in the microstrip line as much as possible. However, the intention of providing a resonator to be used in a band with a high-frequency higher than 3 GHz causes the radiation loss dominant. The resonator using the conventional microstrip line cannot decrease such a radiation loss, then, this fact that a high Q value cannot be achieved in the high-frequency band becomes a subject to be solved.